Yellow and white pavement markings are commonly used on roadways to display traffic lanes. A yellow pavement marking will typically have a different meaning to an automobile driver than a white pavement marking. For example, in the United States of America (USA) a yellow pavement marking is used on a roadway to separate traffic lanes where the traffic moves in opposite directions; whereas a white pavement marking is used (i) to mark the roadway's border at the shoulder, and (ii) to separate traffic lanes where the traffic moves in the same direction (for example, a one-way street). In view of these different functions, it is very important that yellow and white pavement markings are discernible to automobile drivers, particularly at nighttime when visibility is limited. Otherwise, driver confusion may result, creating unsafe driving conditions.
Yellow pavement markings have been made, which are distinctly discernible from white pavement markings under both daytime and nighttime conditions. A typical yellow pavement marking contains clear colorless retroreflective beads partially embedded in a yellow base. The base is made yellow by use of yellow pigments that contain heavy metals such as cadmium, chromium, or lead (see e.g. U.S. Pat. Nos. 2,574,971, 2,268,537, 3,337,483, 4,117,192, 4,248,932, 4,564,556, 4,931,414, Japanese Patent Kokoku 20424/91 and EP 0,305,579 B1). During the daytime, the base diffusely reflects yellow light to display a yellow marking to automobile drivers. At nighttime, the beads reflect light back in the direction from which it came (retroreflect). This retroreflected light is yellow because it strikes the heavy-metal pigments in the base adjacent to the retroreflective beads. The heavy-metal pigments diffusely reflect yellow light back into the beads. The beads then redirect the diffusely scattered yellow light and send it back in the direction of the light source.
Cadmium, chromium, and lead-based pigments have provided good yellow retroreflective pavement markings. Under both daytime and nighttime conditions, the pavement markings are distinctly yellow in appearance. These heavy-metal pigments strongly scatter light because they have a high index of refraction and a particle size on the order of magnitude of the wavelength of light. The pigments provide a distinct yellow color by absorbing the non-yellow components of light to reflect essentially yellow light. This good performance of cadmium, chromium, and lead-based pigments has promoted their widespread use in yellow pavement markings. Chrome yellow (also known as lead chromate) is the most widely used yellow pigment in pavement markings.
It has been known for many years that cadmium, chromium, and lead-based pigments are not environmentally sound. Cadmium, chromium, and lead can be toxic, and therefore replacements have been sought for these pigments. Some states in the USA have announced plans to ban heavy-metals like lead in their pavement markings (A. Banou, Am. Paint & Coatings J. 21-22 (Aug. 19, 1991)). To do so, however, requires that there be suitable replacements for the heavy-metal pigments. The new pavement marking must be highly visible under both daytime and nighttime conditions and must provide a distinct yellow color so as not to be confused with other pavement markings, particularly white pavement markings.
Organic pigments have been recognized as alternatives to heavy-metal pigments (P. Lewis, Organic Pigments, Fed. Soc. for Coatings Tech., Philadelphia, Pa. (October 1988); and J. M. Cameron, Issues and Opportunities in Heavy Metal Replacement, Am. Chem. Soc. Poly. Tech. Conf., Philadelphia, Pa. (June 1991)). Inventors have attempted to use yellow organic pigments in lieu of yellow heavy-metal pigments in pavement markings (see e.g. U.S. Pat. Nos. 3,891,451 and 3,998,645). This attempt has met with little success commercially because organic pigments generally lack strong light-scattering effects.
It has been known to use colored beads in a pavement marking. Yellow-colored beads have been known as early as 1966, as shown in U.S. Pat. No. 3,294,559 to Searight et al. Notwithstanding this long duration of knowledge, yellow beads have not been used in yellow pavement markings to a significant extent. Rather, the pavement markings have relied on colorless beads and yellow, heavy-metal-containing pigments like chrome yellow. Pavement markings have continued to employ these pigments in spite of the long felt need for alternatives.
Most recently, in Japanese Patent Kokoku 20424/91 (published Mar. 19, 1991) a yellow road marking material has been disclosed which contains yellow transparent glass beads, but still employs chrome yellow as a pigment. The glass beads are made yellow by coating them with a film of a thermosetting resin that contains a yellow dye. This patent discloses that the road marking material contains yellow pigments such as chrome yellow, yellow organic pigments, titanium yellow, and yellow iron oxide. This patent also discloses that colored glass beads have also been prepared by melt mixing metal ions such as nickel, chromium, cobalt, or copper in the transparent glass, but discourages the use of such beads because it is difficult to adjust their color and prescribed light absorptivity, and they are expensive.